SPD light valves have been known for more than seventy years for use in the modulation of light. These light valves have been proposed for use in numerous applications during that time including, e.g., alphanumeric displays and television displays, filters for lamps, cameras, optical fibers and displays, and windows, sunroofs, sunvisors, eyeglasses, goggles, mirrors and the like to control the amount of light passing therethrough or reflected therefrom as the case may be. Examples of windows include, without limitation, architectural windows for commercial buildings, greenhouses and residences, windows for automotive vehicles, boats, trains, planes and spacecraft, windows for doors including peepholes, and windows for appliances such as ovens and refrigerators including compartments thereof. Light valves of the type described herein are also known, as indicated above, as suspended particle devices or SPDs.
As used herein, the term “light valve” describes a cell formed of two walls that are spaced apart by a small distance, with at least one said wall being transparent. The walls have electrodes thereon, usually in the form of transparent, electrically conductive coatings. Optionally the electrodes on the walls may have thin transparent dielectric overcoatings thereon. The cell contains a light-modulating element (sometimes herein referred to as an activatable material) which may, without limitation, be either a liquid suspension of particles or a portion of the entire element may comprise a plastic film in which droplets of a liquid suspension of particles are distributed.
The liquid suspension (sometimes herein referred to as a liquid light valve suspension or as a light valve suspension) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement. Hence, a beam of light passing into the cell is reflected, transmitted or absorbed depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state.
For many applications it is preferable for all or part of the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging associated with a high column of light suspension, can be avoided through use of a film and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film, the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A light valve film (also sometimes herein referred to as an SPD film) as used herein means a film or sheet, or more than one thereof comprising a suspension of particles used or intended for use in an SPD light valve. Such light valve film shall comprise either (a) a suspension of particles dispersed throughout a continuous liquid phase enclosed within one or more rigid or flexible solid films or sheets, or (b) a discontinuous phase of a liquid comprising dispersed particles, such discontinuous phase being dispersed throughout a continuous phase of a rigid or flexible solid film or sheet. The light valve film may also comprise one or more additional layers such as, without limitation, a film, coating or sheet or combination thereof, which may provide the light valve film with one or more of, for example, (1) scratch resistance, (2) protection from ultraviolet radiation, (3) reflection of infrared energy, (4) electrical conductivity for transmitting an applied electric or magnetic field to the activatable material, and (5) dielectric overcoatings.
A common (but non-limiting) construction for an SPD film comprises five layers, namely, from one side to the other: (1) a first sheet of polyethylene terephthalate (“PET”) plastic, conveniently 5-7 mils in thickness, (2) a very thin transparent, electrically conductive coating of indium tin oxide (“ITO”) on said first sheet of PET, (3) a layer of cured (i.e., cross-linked) SPD emulsion, usually 2-5 mils in thickness and, (4) a second ITO coating on (5) a second PET plastic substrate. As stated previously, additional layers which provide other functions may optionally be added to the five-layer SPD film described above. Furthermore the SPD film can be laminated, for example, with transparent hot melt adhesive films and/or glass or thicker transparent plastic sheets to provide strength and rigidity and to protect various parts of the combined unit from environmental stresses which may, otherwise, damage its performance characterisitcs.
U.S. Pat. No. 5,409,734 exemplifies a type of non-cross-linked light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking of emulsions are also known. The methods of the present invention are specifically directed to the use of the latter type of film, i.e., film comprising a layer formed by cross-linking an emulsion, and to laminated films produced thereby. See, for example, U.S. Pat. Nos. 5,463,491 and 5,463,492, and U.S. patent application Ser. No. 10/898,303, all of which are assigned to the assignee of the present invention. Various types of SPD emulsions, and methods of curing same, are described in U.S. Pat. Nos. 6,301,040, 6,416,827, and 6,900,923 B2, all of which are assigned to the assignee of the present invention. Such films and variations thereof may be cured through cross-linking brought about by exposing the films to (1) ultraviolet radiation, (2) electron beams or (3) heat. All of the patents and patent applications cited in this application are incorporated herein by reference.
A variety of liquid light valve suspensions are well known in the art and such suspensions are readily formulated according to techniques well-known to one of ordinary skill therein. The term liquid light valve suspension, as noted above, when used herein means a liquid suspending medium in which a plurality of small particles are dispersed. The liquid suspending medium comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
Liquid light valve suspensions useful in the present invention may include any of the so-called prior art liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful herein include, but are not limited to, the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175, 4,407,565, 4,772,103, 5,409,734, 5,461,506, 5,463,492, and 6,936,193 B2, the disclosures of which are incorporated herein by reference. In general one or both of the suspending medium or the polymeric stabilizer typically dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid(s) which comprise the liquid suspending medium. Alternatively, there may be two or more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, when dissolved, provides a plain surface coating for the particles, together with one or more additional types of solid polymeric stabilizer that when dissolved, bond to or associate with the first type of solid polymeric stabilizer and also dissolve in the liquid suspending medium to provide dispersion and stearic protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described for example in U.S. Pat. No. 5,463,492.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light absorbing or light reflecting in the visible portion of the electromagnetic spectrum.
Conventional SPD light valves have generally employed particles of colloidal size. As used herein the term colloidal means that the particles generally have a largest dimension averaging 1 micron or less. Preferably, most polyhalide or non-polyhalide types of particles used or intended for use in an SPD light valve suspension will have a largest dimension which averages 0.3 micron or less and more preferably averages less than one-half of the wavelength of blue light, i.e., less than 2000 Angstroms, to keep light scatter extremely low.
A. Deficiencies of Prior Art SPD Films
Several benefits of using a film as the activatable material for an SPD light valve have been discussed above. However, prior art SPD films also had some significant deficiencies. For example, in prior art films of the type known as poly(organosiloxanes), the layer of cured emulsion therein bonds only weakly to substrates such as the ITO coatings generally used as electrodes on the plastic sheets of PET, which electrodes create an electric field, i.e., a voltage, when powered in a typical SPD film as described above. If the cured emulsion layer bonds only weakly to the ITO, and the SPD film of which it is a part is subjected to shear forces that could result from any of a variety of sources such as but not limited to changes in temperature or pressure, collisions or vibrations, the cured emulsion layer can easily delaminate from one or both ITO coatings, which will often destroy the appearance and proper functioning of the SPD film.
A second deficiency of prior art SPD films is that the film's cohesion varies and may sometimes be less than desired. Lamination of a film whose cohesion is too low may be difficult or unsatisfactory, because when lamination with other layers is attempted at above atmospheric pressure and relatively high temperature, the film may ooze outside its non-laminated area. Such oozing may allow moisture-containing air to enter the gap between the film electrodes near the outer edges of the film, which may be responsible for electrical short circuits when the film is activated.
B. Prior Art Matrix Polymers
Although the inventors do not wish to be bound thereby, it is the opinion of the inventors of the present invention, based on numerous tests and observations, that the two types of prior art film deficiencies described above mainly stem from deficiencies in the film's matrix polymer, which constitutes a majority of the SPD emulsion. The emulsion, after curing, becomes the activatable layer of the SPD film. Weak bonding of a cured emulsion to substrates such as ITO is often referred to as weak or poor adhesion, and is primarily caused by weak adhesion of the matrix polymer to the substrate. Moreover, lower than desired film cohesion described above is primarily caused by weak integrity of the cured emulsion, which also is attributable to the matrix polymer; in the case of low cohesion we have concluded that the problem arises mainly from insufficient cross-linking of the cured emulsion, and said insufficient cross-linking, we have concluded, is due to an insufficient amount of the cross-linking monomer within the matrix polymer.
In order to better understand the present invention, it will be useful to briefly review the recent prior art relating to this area of technology.
U.S. Pat. No. 6,900,923 B2 entitled “Siloxane Matrix Polymers and SPD Light Valve Films Incorporating Same”, which is assigned to the Assignee of the present invention, discloses exemplary films suitable for use as a light-modulating unit of an SPD light valve, and is specifically incorporated herein in its entirety by reference. The present invention is not, however, limited to the specific materials or indices of refraction disclosed in the '923 patent. The film disclosed by the '923 patent comprises a liquid cross-linkable siloxane matrix polymer which has a refractive index >1.4630 and may comprise the main part of an emulsion which, after curing, forms a film layer, which has droplets of a liquid light valve suspension distributed within the cross-linked matrix. Example 1 of U.S. Pat. No. 6,900,923 B2 discloses a method for synthesizing such a matrix polymer. Briefly stated, said matrix polymer is synthesized in the presence of an appropriate solvent and catalyst by performing a condensation polymerization whereby disilanol-terminated dimethyl diphenyl siloxane oligomer, usually referred to as “copodisilanol” herein, is copolymerized with 3-acryloxypropylmethyl dimethoxy silane which is a monomer capable of cross-linking the matrix polymer after exposure to ultraviolet (“UV”) radiation. After the copolymer is formed, it undergoes endcapping, isolation and purification procedures through use of appropriate materials, solvents and equipment in a manner described therein. As would be well understood by one skilled in this art, endcapping a polymer chain can be accomplished by reacting all or substantially all reactive terminal silanol groups in polymer chains with a compound having one functional group, such as trimethyl methoxy silane.
After sandwiching an uncured layer of SPD emulsion comprising the aforesaid (i.e., as disclosed in the '923 patent) matrix polymer and a substantially immiscible liquid suspension in between two ITO-coated PET sheets, the sandwiched parts are then exposed to UV radiation, to cure the emulsion and form a film. If the emulsion layer is well cured, it has been observed to be bonded to the ITO-coated PET substrates, but the bond strength is, however, weaker than may be desired. For those applications utilizing an SPD device comprising such a film, which are not subject to severe environmental stresses, the adhesion of the cured emulsion to the ITO-coated PET may suffice. However, for applications that may involve severe environmental stress, greater adhesion is desirable and may in fact be required for long-term viability. If the adhesion of a cured emulsion to a substrate is insufficient, what one generally observes is a visually objectionable non-uniform area or areas in the film, which non-uniformity results from delamination of cured emulsion from the substrate. The present invention is expected to greatly reduce the chance of such delamination occurring under normal operating and/or storage conditions.
An SPD film in which the cured emulsion adheres relatively strongly to the ITO-coated PET substrates, as in the present invention, is especially useful because such improved adhesion makes it possible to roll up such manufactured films, which facilitates the shipment of substantial quantities of manufactured SPD film to destinations all over the world.